FIGs. 1 and 2 are prior-art schematic diagrams of general purpose drivers 100 and 200 and example load circuits 110. General purpose configurable drivers provide regulated currents for applications such as light-emitting diodes (LEDs), “squibs” (small explosive devices) used in automobile air bag deployment systems, etc.
FIG. 1 illustrates a “high side” configuration, so-called because the load circuit 110 is switched to the high side (e.g., to the drain 112 in this particular example) of the pass element 115. The pass element 115 is typically driven by a preamplifier (e.g., the preamplifier 120) and passes current to the load circuit 110. In an example automobile application, R(LOAD) 125 may represent one or more LEDs, an airbag squib, etc. as previously mentioned. L(WIRING) 130 and C 135 are included in the model of the load circuit 110 to clarify that the load circuit 110 may, generally speaking, be an RLC circuit. The RLC nature of the load circuit 110 has important implications for circuits and methods used to protect the general-purpose drivers 100 and 200 as further described below.
FIG. 2 illustrates a “low side” configuration, so-called because the load circuit 110 is switched to the low side (e.g., to the source 140 in this particular example) of the pass element 115. Although the low side configuration will be used for example purposes throughout this Application, the principles, explanations, and claims apply equally to high side and low side configurations.
FIG. 3 is a prior-art waveform 300 showing transient and steady-state over-current conditions that may occur due to short-circuits to supply or ground rails at a general purpose driver and/or at a load connected to such driver. Over-current conditions that can result in damage to components of the load circuit 110 and/or to the drivers 100 and 200 fall into several categories. The delivered current should be stable when the driver is enabled. When the drivers 100 or 200 are powered on and enabled, a sudden decrease in the value of R(LOAD) 125 could cause excess current flow 310 through the pass element 115 and damage to the latter component. Such might be the case, for example, if R(LOAD) 125 represents a string of LEDs in series and a short to ground occurs somewhere within the series connection. Such a condition could decrease the resistance of R(LOAD) 125, causing the excess current flow 310 through the pass element 115.
When the drivers 100, 200 are in an OFF (disabled) state and if a dynamic short to battery or ground occurs either in the powered state or the unpowered state of the device, the transient energy 312 delivered to the load 110 and/or to the pass element 115 should be limited to prevent either of those components from possible destruction. This translates into a requirement for limiting the peak current level 315 and its duration 320 when the drivers 100, 200 are in the disabled state. And, design steps taken to control the transient energy 312 including the peak transient current 315 must not affect the performance of current limitation circuitry included to protect steady-state over-current conditions described above when the drivers 100, 200 are in the ON (enabled) state.
A transient over-current condition can result when the drivers 100, 200 are either powered on and in the disabled state or powered off. The reason is that the drain-to-gate Miller capacitance 260 of FIG. 2 is typically large for a large power field effect transistor (FET) such as may be used to implement the pass element 115. A short-to-battery condition may cause a voltage spike at the drain of the of the pass element 115. Such a condition may occur, for example, when replacing a car battery and connecting first the negative lead and then the positive lead. The voltage spike may charge the Miller capacitance 260, resulting in a voltage on the input 265 of the pass element 115 and biasing the pass element 115 to a conductive state. In an example case of the driver 200 being a squib driver designed to deploy an air bag under emergency conditions, the driver 200 would normally be powered on but remain in a disabled state under non-emergency conditions. Without transient voltage spike protection, simply connecting the car battery might cause squib firing and air bag deployment.